For a few short years in the 1970s, no one made funk as raw as Betty Davis did. She sang bluntly about sex on her own terms, demanding satisfaction with feral yowls and rasps, her voice slicing across the grooves that she wrote and honed as her own bandleader and producer. Her stage clothes were shiny, skimpy, futuristic fantasies; her Afro was formidable.

A major label, Island, geared up a big national push for her third album, “Nasty Gal,” in 1975. But mainstream radio didn’t embrace her, and Island rejected her follow-up recordings. Not long afterward, she completely dropped out of public view for decades.

Ms. Davis’s voice now — speaking, not singing — resurfaces in “Betty: They Say I’m Different,” an impressionistic documentary that will have its United States theatrical premiere on Wednesday at the Billie Holiday Theater in Bedford-Stuyvesant, Brooklyn, as part of the Red Bull Music Festival. The film includes glimpses of virtually the only known concert footage of Ms. Davis in her lascivious, head-turning prime, performing at a 1976 French rock festival. The present-day Ms. Davis is shown mostly from behind and heard in voice-over, though there is one poignant close-up of her face.

This month Ms. Davis, 72, gave a rare interview by telephone from her home near Pittsburgh to talk about the film and her music. After years of entreaties from and conversations with its director, Phil Cox, and producer, Damon Smith, she agreed to cooperate on “Betty: They Say I’m Different” because, she said, “I figured it would be better to have them cover me when I was alive than when I was dead.”

Mr. Cox said, via Skype from England, “Betty doesn’t want sympathy, and she’s found her own space now. To me, that is just as interesting as that woman she was in the 1970s. It’s the antithesis of the age we live in, where everybody wants to be on social media all the time.”

Ms. Davis has longtime fans from the ’70s and newer ones who have discovered her in reissues and through hip-hop samples. They have clung to a catalog and a persona that were musically bold, verbally shocking and entirely self-created. Long before the current era of explicit lyrics, Ms. Davis was cackling through songs like “Nasty Gal” — “You said I love you every way but your way/And my way was too dirty for you” — and “He Was a Big Freak,” which boasts, “I used to whip him/I used to beat him/Oh, he used to dig it.” She still won’t reveal who was, or whether there was, a real-life model for songs like those.

“I wrote about love, really, and all the levels of love,” she said. That emphatically included sexuality. “When I was writing about it, nobody was writing about it. But now everybody’s writing about it. It’s like a cliché.”

Ms. Davis was born Betty Mabry in Durham, North Carolina, in 1945, and she grew up there and in Pittsburgh. She headed to New York City in the early 1960s, when she was 17, and enrolled at the Fashion Institute of Technology. She supported herself as a model and a club manager; she reveled in the city’s night life, meeting figures like Andy Warhol, Sly Stone, Eric Clapton and Jimi Hendrix.